The chemical industry uses a wide variety of transition metal complexes as catalysts for organic reactions. Polyolefin manufacture is a good example. While conventional Ziegler-Natta catalysts continue to dominate the industry, highly active metallocene or single-site catalysts that give new polymers with narrow molecular weight distributions, low densities, and good comonomer incorporation are emerging.
Transition metal complexes used to polymerize olefins are normally non-zero-valent metals (e.g., Ti.sup.4+, Zr.sup.4+, Sc.sup.3+) surrounded by anionic ligands (e.g., chloride, alkyl, cyclopentadienyl) that satisfy the valency of the metal and often improve the solubility of the catalyst in the reaction medium. The anionic ligands can dramatically impact catalyst activity and polymer properties.
Neutral, multidentate ligands have been used only sparingly in preparing the transition metal complexes useful as precursors for metallocene or single-site polyolefin catalysts. For example, 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) has been used to prepare (tmtacn)MCl.sub.3 complexes of scandium, chromium, and rhodium (see, for example, Wang et al., J. Am. Chem. Soc. 119 (1993) 6999); derivatives of these complexes catalyze olefin polymerizations. Another example is 1,4,7-trithiacyclononane (ttcn), from which (ttcn)RhCl.sub.3 has been prepared (S. Timonen et al., J. Mol. Catal. A., 111 (1996) 267). Because of their complexity, high cost, and synthetic challenge, such ligands have not been widely pursued. While more accessible neutral ligands (such as 1,2-bis(diphenylphosphino)ethane) exist, these have not generally provided significant advantages for metallocene and single-site catalysts.
A potentially viable route to neutral, multidentate ligands reacts three equivalents of pyrazole with chloroform in the presence of a base to give a tris(pyrazolyl)methane. This method is economically attractive because a variety of pyrazoles can be made by reacting acetylacetones with hydrazine. Unfortunately, carbene formation complicates the product mixture and reduces the yield of the desired tris(pyrazolyl) compound.
Anionic tris(pyrazolyl)borate ligands are known. U.S. Pat. No. 5,504,049, for example, reacts VOCl.sub.3 with potassium tris(pyrazolyl)borate to make a complex that polymerizes ethylene in the presence of an activator such as MAO. In addition, one tris(pyrazolyl)silane compound is known: S. Vepachedu et al. (Acta Cryst. C51 (1995) 423)) reported the crystal structure of tris(3,5-dimethylpyrazolyl)methylsilane.
In sum, new neutral, multidentate ligands are needed. Particularly valuable ligands would be easy to synthesize from readily available starting materials. Preferably, the ligands could be made in high yields without complicating side reactions such as carbene formation. Ideally, the ligands would be valuable for making new transition metal complexes useful as procatalysts for olefin polymerization.